Turn the cable box off and turn the display on.
Press and hold Info and Guide on the box itself until a setup menu appears
Select the advanced setup option
Select 16x9 tv type since you have a widescreen tv
Then follow the onscreen instructions to save EVERY output format that you can see correctly on your tv (if you only have EDTV then both 480i's and 480P's, if you have an HDTV then it is the above listed and 720P and 1080i)
Now press power to turn the box back on and go into the normal settings menu
Scroll down to "picture format" or something similar
Select the pass-through option

Now your cable box will not do any scaling or processing on the SD channels so your TV can do it, and will do a better job.